It is well known to carry out the fabrication in the hot state of bituminous coated products from aggregates and molten bitumen in a drying-mixing drum of cylindrical shape mounted for rotation about its axis which is slightly inclined to the horizontal plane so as to permit the circulation of the products in the drum.
The aggregates are continuously introduced through one end of the drum by which the flame of a burner penetrates, so that the drum is traversed by the aggregates and the hot gases circulating in the same direction. The blades disposed inside the drum raise the aggregates and then allow them to fall so that they occupy practically the whole of the transverse section of a drum in at least a part of its length. The molten bitumen is introduced into the drum in such manner as to be mixed with the aggregates, the drum ensuring the drying of the aggregates and their coating by the bitumen. There are recovered at the outlet of the drum coated products ready for use and hot gases which have passed through the drum.
The bituminous coated products must contain, in addition to the aggregates and the bitumen, a certain proportion of fines, i.e., materials having a particle size less than 80 microns. If the starting aggregates include a small amount of fines, it may be necessary to add them as independent constituents at the entrance of the drum. Further, during the drying of the aggregates, the hot gases circulating in the drum have a tendency to separate the fines from the aggregates and to entrain them in the direction of the outlet of the drum where these fines are found in the hot gases escaping in the chimney. It is therefore necessary to provide in the coating installations filters for trapping the fines before allowing the gases recovered in the drum to escape through the chimney of the coating station. The fines recovered by the filters may be re-introduced into the drum so as to maintain the proportion of fines in the coated products at the required value.
Processes for fabricating coated products have been proposed in which the bitumen is introduced onto the aggregates while the latter still enclose practically all their initial humidity. The subsequent drying of the aggregates takes place during the mixing of the aggregates and bitumen which are exposed to the hot gases by raising them in the section of the drum. In such processes, thereare avoided the separation of the fines and their entrainment by the hot gases, these powdered elements being fixed to the aggregates, first by the remaining humidity and then by the bitumen.
There has also been proposed a device described in French Patent Application No. 2,327,048, filed Oct. 8, 1975, which permits substantially complete drying of the aggregates prior to the introduction of the bitumen while avoiding the emission of dust in the gases rejected at the outlet of the drum. For this purpose, the prior drying is effected in a zone of the drum where there is constituted a very dense curtain of materials throughout the section of the drum. The bitumen is introduced just behind the curtain, the mixing being then effected by the rise and fall of the materials throughout the section of the drum. The materials coated with bitumen occupying the whole of the section of the drum enable the fines entrained by the hot gases to be trapped in the drying zone. Thus, there are found at the outlet of the drum gases enclosing a small proportion of fines, and it is possible to guarantee a minimum content of fines in the coated product.
However, in some cases, the initial content of fines of the aggregates is extremely high, and this content exceeds the desired final content in the coated products.
The processes and devices of the prior art described hereinbefore do not permit a reduction in the content of fines of the aggregates so as to maintain the optimum composition of the coated products.
Processes and devices are known which permit reutilizing the coated products recovered on repaired roads after crushing. Such devices may comprise a ring for introducing recycled materials obtained by crushing or milling in the vicinity of the central part of the drum. It has been proposed to extract the excess fines in the region of the recycling ring by removal of a part of the circulating gases. Such a process, however, has the drawback of reducing the yield of the installation and preventing its recycling operation when the device for extracting the gases in the recycling ring is put into operation.